1. Field
Embodiments of the present invention generally relate to signal processing and, more particularly, to a method and apparatus for combining digital signals.
2. Description of the Related Art
In modern communications systems, digital signal processing is used. Such processing may require a plurality of digital signals to be combined to form a composite signal. For example, audio signals are typically encoded as pulse code modulation (PCM) signals, where a signal is represented as a sequence of integer values. Audio signal combination (mixing) is used to combine multiple live or recorded channels of audio signals to produce a single channel. There are many scenarios in which audio mixing is needed, for example, in telephonic conferencing, Voice over Internet Protocol (VoIP) conferencing, video conferencing, and the like, where multiple participants may be speaking and all spoken audio is combined for each participant to hear through their telephone handset or computer.
Conventional methods of digital audio mixing generally involve superposition. In one such method, temporally synchronized samples of a plurality of digital signals are added arithmetically and the result of the summation forms an output sample. However, this method of signal combination creates distortion (‘audio clipping’) due to limited dynamic range of sound reproduction devices attempting to reproduce combined signals having large amplitude. Additionally, using a superposition-based signal combiner, the output signal may include a substantial amount of unfiltered noise. For example, an increase in the number of input signals during a conference bridge (i.e., a new caller is added to a conference call) may cause an increase in the amplitude of output samples. Because the additional noise from the additional caller adds to the output signal, even when participants on the call are silent, the total noise heard by the participants is increased. Such cumulative noise creates a significant background “hiss” that is heard by all participants.
To compensate for the above mentioned distortion, various post processing techniques, such as dynamic range compression, automatic gain control, fixed amount of reduction in decibel gain depending on number of channels added, are generally applied to the output signal. Depending upon the complexity of practical implementation, the previously mentioned solutions tend to increase the computational cost, increase hardware cost and are generally ineffective at controlling noise.
Therefore, there is a need for an improved method and apparatus for combining digital signals.